1. Field of the Invention
The present invention relates to method for fabricating a semiconductor device and more particularly, with a method for fabricating a capacitor of a semiconductor device which is especially suitable for a highly-integrated semiconductor device.
2. Description of the Prior Art
Generally, in fabricating a semiconductor device, the devices are implemented in higher integration and so the cells become smaller. Consequently securing sufficient capacitance in proportion to the surface area of the storage electrode has become a big smaller.
Particularly, in a DRAM in which a unit cell is comprised of a single MOS transistor and a capacitor, it is important for the high integration of the DRAM to reduce the area of the capacitor which takes a large part of the area in the chip at the same time with enlarging the capacitance of the capacitor.
In the meantime, the capacitance C is calculated by an equation, EQU C=(E0.multidot.Er.multidot.A)/T,
in which E0, Er, A, and T mean respectively, a vacuum dielectric constant, a dielectric constant of a dielectric film, an area of the capacitor, and a thickness of the dielectric film, that is, the larger the specific dielectric constant Er of the dielectric film and the lesser the thickness T of the dielectric film, the larger the capacitance of C. Therefore, to achieve the high integration of the semiconductor device, it is preferred that the dielectric film is thin and made from a material having a high dielectric constant.
However, in the case in which the dielectric film is thin and made from a material having a high dielectric constant, the electric characteristic of the device becomes unstable and the reproduction characteristic thereof deteriorates due to the hillock and pin hole phenomena which occurs on a surface of the lower electrode to form a capacitor.
Therefore, a process has recently been proposed to absolve the above problem. In this process, an upper electrode and a lower electrode are formed by means of ruthenium oxide (RuO.sub.2), and then their operational characteristic is stabilized by heat treatment.
The conventional process will be described hereinafter with reference to FIGS. 1A to 1C which are schematic cross sectional views of a semiconductor device for showing the steps of the conventional process.
First, a lower insulating layer 2 is formed on a semiconductor substrate 1 as shown in FIG. 1A. Thereafter, the lower insulating layer 2 is selectively etched by means of a contact mask (not shown) for a capacitor so that a contact hole 3 for exposing that predetermined portions of the semiconductor substrate 1 is formed.
Thereafter, a polycrystalline silicon film (not shown) is formed on the entire exposed surface, and then the polycrystalline silicon film is etched so as to form a plug 4 in the contact hole 3.
Then, as shown in FIG. 1B, a first ruthenium oxide (RuO.sub.2) film 5, to be used as a lower electrode of the capacitor, is formed on the entire plug 4 and a part of the lower insulating layer 2, in the vicinity of the plug 4. Thereafter, dielectric material is deposited on the first ruthenium oxide film 5 so as to form a dielectric film 6.
Thereafter, as shown in FIG. 1C, a second ruthenium oxide (RuO.sub.2) film 7, to be used as an upper electrode of the capacitor, is formed on the dielectric film 6 so that the manufacture of the capacitor is completed.
In this case, the first and the second ruthenium oxide films 5 and 7 are formed by means of ruthenium target and oxygen gas or according to a process in which ruthenium is deposited by the chemical vapor deposition (CVD) method and is then oxidized in a tube through annealing treatment.
The following is a discussion of the problems of the conventional method for fabricating a capacitor of a semiconductor device, as described above.
In the conventional method for fabricating a capacitor of a semiconductor device, in case where the ruthenium oxide films are formed by means of ruthenium target and oxygen gas, stable ruthenium oxide films are difficult to form due to the low oxidation speed of the ruthenium.
Furthermore, when partial pressure of the oxygen gas is high, RuO.sub.3 gas or RuO.sub.4 gas, which has a strong volatility, is generated, and thus, the thin film is volatilized.
Moreover, in a case where ruthenium is deposited by the CVD method and is then oxidized in a tube through the annealing treatment when the ruthenium oxide film (RuOx) is formed in the conventional method for fabricating a capacitor of a semiconductor device, RuO.sub.3 gas or RuO.sub.4 gas, which has a higher volatility than that of the ruthenium oxide film, is generated when the ruthenium is oxidized in the tube, and thereby the tin film is volatilized.
Therefore, in the conventional method for fabricating a capacitor of a semiconductor device, due to the phenomenon that the thin film is volatilized, a desirable capacitor is not formed. Furthermore, the reliability and the operational characteristic of the semiconductor device deteriorates.
In other words, because the thin film is volatilized and degraded due to oxide generated during the formation of the pattern, the reliability and the operational characteristic of the semiconductor device deteriorates.
Therefore, the conventional method for fabricating a capacitor of a semiconductor device is not suitable for the high integration of the semiconductor device.